The present invention relates to spectrum analyzers and, more particularly, to a calibrated radio frequency analog spectrum analyzer.
Analysis of signals in the frequency domain is widely used to obtain physical and electrical system performance information. For example, manufacturers of mechanical structures, such as aircraft and bridges, can use a motion-to-electrical signal transducer. A spectrum analysis of the resulting signal can permit monitoring of vibration components associated with imbalance and worn bearings and gears. In addition, a system's natural modes of vibration can be identified.
Spectrum analyzers are also used in electronic testing to assess non-linear effects of amplification, mixing and filtering, to determine the purity of signals, to measure radio frequency power, frequency and modulation characteristics, and to provide amplitude analysis of electrical networks. In telecommunications, transceivers and multiplex systems are assessed with respect to their spectrum, modulation, wave and audio characteristics.
Of central concern herein, is the use of spectrum analyzers for determining the spectral components of a radio frequency (rf) signal. A communications system, for example, can incorporate such a spectrum analyzer to transform a frequency division multiplexed (FDM) signal to a time division multiplexed (TDM) signal.
Spectrum analysis can be performed digitally by employing a Fourier transformation. For some applications, the computational power requirements for a desired input bandwidth are impractical. This is particularly true in satellite communications where system power and weight are severely constrained. In addition, the digital processing introduces undesirable delays. Furthermore, it is difficult to build sufficiently high speed analog-to-digital converters.
Analog spectrum analyzers offer the prospect of near real time transformations without great demands on processing power. Traditional analog spectrum analyzers either employ filter banks or swept filters. Filter banks are very bulky while swept filters are unsuitable for some applications such as demodulators where it is necessary to sample each frequency continuously.
Analog spectrum analyzers are known which include a pair of frequency x delay dispersion sections with an intervening frequency sweep section. The first dispersion section introduces delays as a function of frequency to an incoming signal, such as a FDM signal. The frequency sweep signal converts the frequency components of the dispersed FDM signal into a series of sweeps. The second dispersion section collapses each sweep into a pulse so that the series of sweeps becomes description of the spectrum of the input signal.
The dispersion sections in such devices include devices generically classified as dispersive filters. These filters are also known as surface acoustic wave (SAW) dispersive or linear frequency modulated chirp filters. The first such devices were based on interdigital electrode transducers (IDTs). The IDT consists of a set of interleaved metal electrodes deposited on the surface of a set of piezoelectric substrate, normally quartz. However, IDT based spectrum analyzers have not provided the time-bandwidth product sufficient for some satellite communications applications.
More recently, devices with greater time-bandwidth products have been provided using reflective array compressors (RACs). The RAC can be manufactured by etching into a crystalline substrate, e.g. lithium niobate, a multitude of slits, e.g. 8,000, each tuned to reflect a given frequency. By reflecting different frequencies at different slits, and thus different locations, differential delays are introduced in a throughgoing signal as a function of frequency.
RACs are difficult to manufacture with the precision required for satellite applications. Furthermore, the cost of manufacturing a single RAC renders impractical the discarding of RACs not meeting design specifications. What is needed is an analog spectrum analyzer with the time-bandwidth product of RAC based devices, but without vulnerability to RAC imperfections.